Hideous adder left guardedly when confused (4)
I believe the answer is:
ugly
'hideous' is the definition.
(similar in meaning)
'adder left guardedly when confused' is the wordplay.
'left' is a deletion indicator.
'when confused' is an anagram indicator (I've seen this in other clues).
'guardedly' with 'adder' taken away is 'guly'.
'guly' anagrammed gives 'UGLY'.
(Other definitions for ugly that I've seen before include "Threatening; nasty" , "Hideous" , "Unprepossessing, repulsive" , "Not beautiful, plain" , "Unpleasing to the eye" .)